The field of the invention relates to navigational systems and more particularly to global positioning systems.
Global navigation satellite systems (GNSS), such as the US NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Defense Department is a satellite-based radio navigation system that transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with equal precision. The GPS uses 24 satellites dispersed in six, inclined, 12-hour circular orbits chosen to insure continuous 24-hour coverage world wide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHZ and an L2 signal at a frequency of 1227.6 MHZ. The L1 and L2 signals are bi-phase modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigational data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN coded signal, a receiver decodes the signal to recover the navigational data, including emphemeris data. The emphemeris data is used in conjunction with a set of Kepler equations to precisely determine the location of each satellite. The receiver measures a phase difference (e.g., time of arrival) of signals from at least four satellites. The time differences are then used to solve a matrix of at least four equations to provide a space and time solution. The result is a precise determination of location of the receiver in three-dimensional space.
In addition to processing of the PRN codes to determine location, other techniques such as differential GPS positioning can be used to improve location accuracy of a mobile station by transmitting correction information from a reference station. Differential GPS can be used to correct the position solution of a mobile station by transmitting a correction signal to the mobile station from a reference station with a precisely know position. Kinematic GPS is a precise form of differential GPS positioning using post-processed GPS carrier phase data to determine the relative position of a mobile station with respect to a reference station.
In differential and kinematic GPS system, measurements made by the mobile station receiver must be time matched to those made by the reference station receiver to produce a single solution of position of the mobile station. In a real time system there is latency in the position solution computed from the pairing of measurements resulting from delays encountered in transporting the data from the reference station site to the mobile site. If the solution rate needs to be increased, the transport rate must also be increased accordingly. A real time implementation of the differential and kinematic GPS system would require a high bandwidth data link to minimize solution latency and accommodate high rate solution computations. The problem is then one of how to obtain a solution with minimal latency that may also be produced at high rates, while only using a low-bandwidth data link.
The present invention is directed to a method and apparatus for deriving high-rate data output with minimal latency when using a low-bandwidth data link for transporting code and carrier phase corrections or code and carrier phase measurements in a real-time differential and kinematic GPS system. What is disclosed is a method and apparatus for deriving a position solution in a global positioning system mobile station. The method includes forming a position solution in the mobile station during a first time period based upon a first set of signal measurements made by the mobile station of signals from a set of global positioning system satellites and a corresponding first set of satellite signal measurements made by a ground-based reference station at a known location remote from the mobile station. A second position solution is formed by the mobile station during a second time period based upon a second set of satellite signal measurements made by the mobile station of the set of satellites during the second time period and the corresponding first set of measurements from the reference station.
It is therefore an object of the present invention to provide a global positioning system with improved accuracy by the use of differential and kinematic techniques.
It is another object of the present invention to provide the improved accuracy at high rate.
It is a feature of the present invention to provide the improved GPS accuracy at a high rate without the use of wide bandwidth data links to transmit the GPS differential and kinematic correction data.
It is an advantage of the present invention to use low bandwidth data links such as cellular telephones to transmit the GPS differential and kinematic correction data from a reference station to a mobile station.
These and other objects, features, and advantages are disclosed and claimed in the specification, figures, and claims of the present application.